Inhibition of alkaline hydrolysis of ethyl p-nitrophenyl (chloromethyl)phosphonate in the system cationic surfactant-water-electrolyte

Article abstract of DOI:10.1023/A:1020867426981

The rate of alkaline hydrolysis of ethyl p-nitrophenyl (chloromethyl)phosphonate in micellar solutions of cationic surfactants decreases with increasing concentration of chloride, bromide, and salicylate ions. In the presence of electrolytes, the critical micell concentration of surfactants and the surface potential of micelles also decrease.

Full text of DOI:10.1023/A:1020867426981

Russian Journal of General Chemistry, Vol. 72, No. 8, 2002, pp. 1215 1221. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 8, 2002,
pp. 1296 1302.
Original Russian Text Copyright
2002 by Zakharova, Kudryavtsev, Valeeva, Kudryavtseva.
Inhibition of Alkaline Hydrolysis
of Ethyl p-Nitrophenyl (Chloromethyl)phosphonate
in the System Cationic Surfactant Water Electrolyte
L. Ya. Zakharova, D. B. Kudryavtsev, F. G. Valeeva, and L. A. Kudryavtseva
Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center,
Russian Academy of Sciences, Kazan, Tatarstan, Russia
Kazan State Technological University, Kazan, Tatarstan, Russia
Received October 20, 2000
Abstract The rate of alkaline hydrolysis of ethyl p-nitrophenyl (chloromethyl)phosphonate in micellar
solutions of cationic surfactants decreases with increasing concentration of chloride, bromide, and salicylate
ions. In the presence of electrolytes, the critical micell concentration of surfactants and the surface potential
of micelles also decrease.
As known, nucleophilic substitution in carboxylic
O
and phosphorus acid esters is accelerated by cationic
micelles [1 3]. The catalysis is explained by solubili-
zation of hydrophobic substrates in the surface layer
of micelles, where hydroxide ions are also concentra-
ted due to electrostatic attraction to the positively
charged micellar surface. Electrolytes reduce the
catalytic effect [4, 5]. However, together with trivial
inhibition, electrolytes can indirectly affect reaction
rates in micellar surfactant solutions. Increased con-
centrations of counterions change many micellar
characteristics, producing decrease in the critical
micell concentration (cCMC), increase in the aggrega-
tion numbers and molecular weights of micelles, etc.
At certain critical concentration of electrolytes, the
shape of micelles changes from spherical to cylindri-
cal [6, 7]. The mentioned changes in micellar
parameters may affect the reactivity of compounds
in micelles.
ClCH2
C2H5O
P O C6H4NO2-p + OH
O
ClCH2
P O + OC6H4NO2-p.
C2H5O
effect which is the main driving force of micellization
in aqueous surfactant solutions. The cCMC values in
the absence of electrolytes are 0.062, 0.011, 0.0084,
and 0.0085 M for compounds I IV, respectively. As
seen from Figs. 1 and 2, increasing electrolyte con-
centration reduces cCMC. The observed effect is ex-
plained by partial neutralization of the surface charge
of micelles with increasing concentration of surfactant
counterions, and the resulting attenuation of the de-
stabilizing electrolytic repulsion of the likely charged
head groups and reduction of the Gibbs energy of
micellization. The cCMC values depend not only on
electrolyte concentrations, but also on the nature of
surfactant counterions.
In the present work we studied the kinetics of al-
kaline hydrolysis of ethyl p-nitrophenyl (chloro-
methyl)phosphonate in micellar solutions of cationic
surfactants [decylpyridinium chloride (I), dodecyl-
pyridinium bromide (II), cetylpyridinium bromide
(III), and cetyltrimethylammonium bromide (IV)] in
the presence of inorganic and organic electrolytes
(NaCl, NaBr, and NaSal). Simultaneously, by surface
tension and electroconductivity measurements we
determined cCMC for these solutions.
Highly hydrophilic counterions produce a weaker
effect on micellization; vice versa, less hydrophilic
counterions stronger bind with micellar surface and
stronger affect the properties of micellar aggregates.
Organic counterions are, as a rule, more effective than
inorganic. We found the following activity order of
counterions (in terms of their effect on the micelliza-
tion process): Cl < Br < < Sal .
Figures 1 and 2 represent the resulting cCMC values.
As seen, cCMC decreases with increasing chain length
in the surfactant, on account of enhanced hydrophobic
With reference to data in [8], we obtained empirical
dependences of the cCMC values for surfactants I III
on the concentration of various electrolytes, which
1070-3632/02/7208-1215$27.00 2002 MAIK Nauka/Interperiodica

1216
ZAKHAROVA et al.
(a)
60
50
800
600
400
200
(a)
1
0
2
40
3
0.005 0.010 0.015 0.020 0.025 0.030
(b)
4
250
0
0.04
0.08
0.12
0.16
(b)
200
150
100
50
45
0
0
0.001
0.002
(c)
1
40
2
3
200
160
120
0
0.02
0.04
0.06
0.08
(c)
55
0.001
0.002
0.003
50
45
150
1
2
(d)
100
50
3
40
0
0.001
0.002
0.003
cI, M
0
0.0005 0.0010 0.015
0.0020
cI, M
Fig. 1. Dependence of the surface tension ( ) of aqueous
solutions of compound I on its concentration at various
Fig. 2. Dependence of the electroconductivity ( ) of
aqueous solutions of compound IV on its concentration
electrolyte concentrations (25 C). (a): c
(2) 0.1, (3) 0.2, and (4) 0.4 M; (b): c
, (1) 0,
(1) 0.04,
NaCl
at various electrolyte concentrations (35 C).
c
:
NaBr
salt
(2) 0.1, and (3) 0.2 M; and (c): c
(2) 0.002, and (3) 0.005 M.
(1) 0.0005,
(a) 0 and (b) 0.005 M NaCl, (c) 0.001 M NaNO , and
(d) 0.0002 M NaSal.
NaSal
3
allow cCMC at any electrolyte concentration to be
estimated. Since the micellization patterns of com-
pounds III and IV in aqueous solutions are similar,
we studied in detail association of the amphiphilic
compound III only.
[9]. In this connection many authors use cCMC in
deducing theoretical and empirical equations for
estimation of different micellar parameters.
Hobson et al. [10] deduced a Nernst relation
between the suface potential of micelles and cCMC
:
I:
ln cCMC
ln cCMC
ln cCMC
=
=
=
0.46ln cNaCl
0.35ln cNaBr
0.16ln cNaCal
5.39
4.54
7.59
d dlog cCMC = 59.16 mV, which allows calcula-
tion of the potential of micelles at any salt concentra-
tion (Tables 1 and 2). Of particular interest is to trace
trends in the potential
of cationic micelles in the
II:
III:
ln cCMC
ln cCMC
ln cCMC
=
=
=
0.41ln cNaCl
0.35ln cNaBr
0.16ln cNaCal
6.45
6.95
12.27
presence of salicylate ion. Whereas the mechanism
of action of inorganic electrolytes is well-understood,
and a great number of models for interpreting their
effect are proposed, the unique behavior of Sal ion
has scarcely been explored. According to published
[11 13] and our data [14], this ion behaves much
differently from hydrophilic inorganic counterions and
is much more effective than aliphatic and aromatic
electrolytes, as well as its close homologs, m- and
ln cCMC
ln cCMC
=
=
0.46ln cNaCl
0.35ln cNaBr
5.39
4.54
The cCMC value is a basic parameter of micelliza-
tion, which reflects balance of various thermodynamic
contributions into the Gibbs energy of micellization
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 8 2002

INHIBITION OF ALKALINE HYDROLYSIS
1217
Table 1. Values of cCMC and
for micelles of surfactants I and II at various concentrations of counterions
Cl
Br
Sal
csalt
,
cCMC
,
,
csalt
,
cCMC
,
,
csalt
,
cCMC
,
,
M
M
mV
M
M
mV
M
M
mV
Compound I
0.030
0.05
0.07
0.1
0.2
0.3
0.5
1.0
1.5
0.055
0.046
0.038
0.026
0.021
0.016
105.7
101.1
96.2
86.4
81.0
74.0
65.3
61.2
0.05
0.07
0.1
0.2
0.3
0.5
1.0
1.5
90.1
87.4
84.4
78.4
74.0
70.5
64.3
59.2
0.001
0.002
0.005
0.01
0.02
0.03
0.0015
0.0014
0.0012
0.001
0.0009
0.00088
0.00085
0.00082
13.2
11.4
7.4
2.7
0.04
0.5
1.4
2.3
0.027
0.024
0.019
0.016
0.014
0.011
0.009
0.0115
0.0096
0.04
0.05
Compound II
0.0076
0.0056
0.0046
0.0041
0.0037
0.0032
0.0027
0.0020
0.0016
0.0013
0.01
0.02
0.03
0.04
0.05
0.07
0.1
0.2
0.3
0.5
1.0
0.010
123
116
112
109
107
103
99
92
88
82
73
0.01
0.02
0.03
0.04
0.05
0.07
0.1
0.2
0.3
0.5
121
0.001
0.002
0.005
0.01
0.02
0.03
0.04
0.05
0.07
0.0017
0.0009
0.0004
0.0002
0.0001
0.00009
0.00007
0.00006
0.000045
77
0.0078
0.0066
0.0059
0.0054
0.0047
0.0041
0.0030
0.0026
0.0021
0.0016
0.0013
112
107
103
100
95
91
82
76
71
62
41
26
11
1.8
4
9
13
1.5
70
p-hydroxybenzoic acids. In the presence sodium
salicylate, not only cCMC values sharply decrease, but
also the physical properties of the solution are much
affected, in particular, its viscosity considerable in-
creases, and a viscoelastic state develops, charac-
teristic of dilute solutions of polymers. Manohar et al.
[15] suggest that the action of Sal is largely underlied
by orientation of ions in the surface layer of micelles,
which ensures particle cohesion and structuring of
micellar solutions, i.e. the aggregates acquire new
physical characteristics.
Table 2. Values of cCMC and
III at various concentrations of counterions
for micelles of surfactant
Cl
Br
csalt
,
cCMC 103,
,
csalt
,
cCMC 103,
,
M
M
mV
M
M
mV
0
0.80
0.79
0.64
0.57
0.52
0.49
0.44
0.41
0.32
0.27
0.24
140
139
134
131
129
128
125
123
117
112
110
0
0.80
0.75
0.46
0.34
0.28
0.24
0.19
0.15
0.09
0.069
0.048
140
137
124
116
111
107
101
95
0.01
0.02
0.03
0.04
0.05
0.07
0.1
0.2
0.3
0.5
0.01
0.02
0.03
0.04
0.05
0.07
0.1
0.2
0.3
0.5
Figure 3 represents the kinetic data for alkaline hyd-
rolysis of ethyl p-nitrophenyl (chloromethyl)phospho-
nate in micellar solutions of surfactants I IV, and Tab-
le 3 lists the results of quantitative analysis of the kapp
csurf dependences in terms of the pseudophase model
of micellar catalysis with use of Eqs. (1) and (2) [1].
82
75
66
k2,m
V
k2,w
+
KSKOHcsurf
(1)
kapp
=
.
(1 + KScsurf)(1 + KOH surf)
c
first-order rate constant (kapp) by the total nucleophile
1
1
Here kapp (l mol 1 s ) is the second-order rate
concentration]; k2,w and k2,m (l mol 1 s ) are the
constant [obtained by dividing the apparent pseudo-
second-order rate constants in the aqueous and
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 8 2002

1218
ZAKHAROVA et al.
Table 3. Kinetic data [Fig. 3, Eq. (1)]
Comp.
no.
k2,w
,
KS,
KOH
,
k2,m,
l mol 1 s
(kapp/kw)max
Fc
Fm
FcFm
l mol 1 s
l mol
l mol
1
1
1
1
I
II
III
IV
4.0
4.0
4.0
4.0
4.8
9.5
35
17.1
219
2020
1775
6.6
4.2
80
240
2.8
4.0
0.78
0.055
8.4
10.8
187
432
0.7
1.0
0.19
0.014
5.9
10.8
36
7.5
6
1
micellar phases, respectively; KS, KOH (l mol ) are
the substrate and nucleophile binding constants; V is
the molar volume of surfactant; and csurf is the con-
centration of surfactant minus cCMC. Using the trans-
formed form (2) of Eq. (1) allows two major factors
reaction acceleration equal to the ratio of the apparent
pseudo-first-order rate constant to the pseudo-first-
order rate constant in water (kw). The Fm value is
equal to the first multiplier in the right-hand part of
Eq. (2) and relates to the change in rate constant, pro-
responsible for the micellar effect to be revealed: con- duced by transfer of reagents from bulk pseudophase
centrating (Fc) and micellar microenvironment (Fm).
to micellar and change in their microenvironment
(micropolarity, solvation shell, etc.); the Fc value is
equal to the second multiplier in the right-hand part
and relates to the catalytic effect produced by the local
increase of the concentrations of reagents on their so-
lubilization in the micellar pseudophase whose volume
is much lower that the volume of solvent pseudophase.
k2,m
k2,w
KSKOH
V(KS1/2 + K
(2)
(kapp/kw)max
=
.
1/2 2
)
OH
Here the term in the left-hand side is the maximum
(a)
As the surfactant chain length increases, the cata-
lytic effect enhances, and the maximum acceleration
factor (kapp/kw)max increases 7-fold in going from
surfactant I to surfactant II (Table 3). This result is
explained by the sharp increase in Fc, resulting from
increased substrate bonding constant: KS increases
more than two orders of magnitude as the surfactant
chain length increases from 10 to 16 carbon atoms.
The latter result is explained by the more effective
micellization in solutions of compound III, lower
cCMC values, and coarser aggregates with a higher
solubilizing capacity as compared with lower homo-
logs. A strong reactivity effect of end groups of sur-
factants is also noted: Micelles of compound III are
5 times catalytically more active than micelles of
compound IV. The reason for the latter effect lies in
the reduced reactivity of the substrate in the micellar
pseudophase of compound IV. As seen from data in
Table 3, k2,m in micelles of compound III is more
than an order of magnitude higher of the respective
value for compound IV.
1.5
1.0
0.5
0
0.1
0.2
0.3
(b)
4
3
2
1
0
0.02
0.04
0.06
0.08
(c)
8
6
1
4
2
2
0
In the context of the experimental task, we studied
hydrolysis of ethyl p-nitrophenyl (chloromethyl)phos-
phonate in the presence of electrolytes and treated the
kinetic data on semilog coordinates (Fig. 4). It was
found that kapp decrease with increasing concentration
of counterions in solutions.
0.005
0.010
0.015
0.020
[Surfactant], M
Fig. 3. Dependence of the apparent rate constant (k
)
app
of alkaline hydrolysis of ethyl p-nitrophenyl (chloro-
methyl)phosphonate on the concentration of surfactant.
(a) Compound I, (b) compound II, and (c) compounds
The mechanism of such inhibition we have ana-
lyzed in [16]. It was shown that with different re-
(1) III and (2) IV (c
0.001 M, 25 C).
NaOH
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 8 2002

INHIBITION OF ALKALINE HYDROLYSIS
1219
(a)
agents two different inhibition mechanisms can be
0.12
0.08
0.04
0
1
operative. One of them involves reduction of the
surface potential of micelles and the resulting weaken-
ing of the electrostatic interaction of hydroxide ions
with the positively charged sufrace. The other in-
volves substrate expulsion from the surface micellar
layer. As seen from Fig. 4, the effect of counterions
the kinetics of hydrolysis of the phosphonate in study
varies in parallel with their effect on the micellization
process: Cl < Br < Sal . The surface potentials listed
in Tables 1 and 2 provide evidence showing that the
charge of micelles decreases with increasing concentra-
tion of counterions in the solution, and the effect
enhances in going from Cl to Br . As concerns sali-
cylate ions, then when their concentration attains
some defined value, charge reversal of micelles takes
place. These data are consistent with data of Magid
et al. [12] and Imae and Koshaka [13], who, too,
2
3
2.0 1.6 1.2 0.8 0.4 0.0 0.4
(b)
0.15
0.10
0.05
0
3
2
1
5
4
3
2
1
0
0.12
0.10
(c)
1
0.08
reported negative values of
potential in the presence of NaSal.
and electrokinetic
0.06
0.04
0.02
3
2
As seen from Fig. 4, the kapp logcsalt plot has an
inflex at some critical point. Analysis of our previous
data [17] and data in [6] led us to conclude the critical
concentrations (ccr) in Fig. 4 correspond to the elec-
trolyte-induced sphere rod micellar transitions.
This conclusion was based on the following reasoning.
0
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0.5
log csalt
Fig. 4. Dependence of the apparent rate constant (k
)
app
First, according to published data [6, 7], increase of
the concentration of surfactant counterions in a
micellar solution with a concentration close to the
critical micell concentration increases cCMC and the
size of micelles. Near cCMC micelles has a spherical
shape, and their radius is no larger than the length of
a gaunt surfactant molecule. The shape and size of
micelles are determined by a combination of geo-
metric and energetic criteria [18], and, therefore, in
the course of micelle growth at a certain counterion
concentration the spherical shape of micelles may
come into collision with these criteria. The energetic
and geometric factors turn more favorable for rod-
shaped aggregates in which micelles may elongate
practically without limit. The sphere rod micellar
transition occurs, which has been proved experimental-
ly by quasielastic light scattering, small-angle neutron
scattering, viscometry, etc. [6, 7, 19, 20].
of alkaline hydrolysis of ethyl p-nitrophenyl (chloro-
methyl)phosphonate on the logarithm of electrolyte
concentration. (a) Compound II: (1) NaCl, (2) NaBr,
and (3) NaSal; (b) compound III: (1) NaCl, (2) NaBr,
and (3) NaSal; and (c) compound I: (1) NaBr, (2) NaSal,
and (3) compound IV, NaSal (c
0.005 M and c
surf
NaOH
0.001 M, 25 C).
and inorganic electrolytes. The activity order of
counterions for all the processes and surfactants studied
was found to be the same: F < Cl < Br
NO2 <
Sal < Tos . As seen from Fig. 4, the critical concen-
tration depends on the nature of the surfactant and
shifts to lower values for compounds with greater
number of methylene units. Given are surfactant and
ccr (M) for Cl , Br , and Sal , respectively: I, , 1, 0.1;
II, 1, 0.5, 0.05; III, 0.1, 0.02, 0.0015; and IV, 0.1,
0.04, 0.0015. This phenomenon appears to be ex-
plained by different mechanisms of micellization in
solutions of the listed surfactants. As noted above,
cCMC decreases in the order I > II > III IV and
is 5 8 times lower for each successive compound.
Therefore, the concentration of the micellar solution
of compound I near cCMC is much higher that for
compounds II and III, and for partial neutralization
of the charge of head groups of surfactant I requires a
higher electrolyte concentration. With compounds III
and IV, we obtained the same ccr values for Sal ion.
Second, we have performed quite a comprehensive
kinetic experiment here and in previous works
[14, 17]. In the present work we used four cationic
surfactants with different chain lengths and head
groups. Earlier [17] we stidued three different pro-
cesses: alkaline hydrolysis p-nitrophenyl acetate and
bis(p-nitrophenyl) methylphosphonate and acid base
dissociation bis(chloromethyl)phosphinic p-nitro-
anilide in the micellar colution of compound IV. In all
the experiments we used the same series of organic
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 8 2002

1220
ZAKHAROVA et al.
This result provides one more evidence to show that
critical concentrations relate to the micellization
process rather than to kinetics, as ethyl p-nitrophenyl
(chloromethyl)phosphonate has much different re-
activities in the micellar solutions of compounds III
and IV (Table 3), whereas the corresponding cCMC
values are quite close. As shown in [17], all the three
hydrolysis and acid base dissociation processes in
micelles of compound IV have coincident ccr values
for each of the couterions. The aforesaid means that
ccr depends not only on the nature of counterion and
surfactant and is independent on the type of the pro-
cess, i.e. is a characteristic of a surfactant with each
concrete counterion.
Ethyl p-nitrophenyl (chloromethyl)phosphonate
was synthesized as described in [22]. Pure grade sur-
factants I III were twice recrystallized from ethanol.
Compound IV (Sigma) was used as received.
Kinetic measurements were performed on a
Specord M-400 spectrophotometer at 25 C, following
the absorbance of p-nitrophenolate anion at
400 nm. The initial concentration of the substrate in
5
kinetic experiments was 5 10 M. The apparent
pseudo-first-order rate constants (kapp) were deter-
mined from the dependence ln (D
D) = kapp +
const (D and D are the optical density of the solution
at time and at reaction completion). The kapp values
were calculated by the least-squares method.
Third, it was found that the ccr values for micelles
of compounds III and IV, measured by physical
methods in [19], and the kinetic ccr values we ob-
tained in [17] coincide. We perfomed a parallel kinetic
study of hydrolysis of ethyl p-nitrophenyl (chloro-
methyl)phosphonate in micelles of compound II and
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Foundation for Basic Research (project no. 99-03-
32037a) and Khal’dor Topse Joint-Stock Company.
1
of micellization of the latter compound by H NMR
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